1. Field of the Invention
The present invention relates to an exposure apparatus having a projection optical system for projecting a pattern of a first object onto a photosensitive substrate or the like as a second object, and more particularly to a projection optical system suitably applicable to projection exposure of a pattern for semiconductor or liquid crystal formed on a reticle (mask) as the first object onto the substrate (semiconductor wafer, plate, etc.) as the second object.
2. Related Background Art
As the patterns of integrated circuits become finer and finer, the resolving power required for the exposure apparatus used in printing of wafer also becomes higher and higher. In addition to the improvement in resolving power, the projection optical systems of the exposure apparatus are required to decrease image stress.
Here, the image stress includes those due to bowing or the like of the printed wafer on the image side of projection optical system and those due to bowing or the like of the reticle with circuit pattern written therein, on the object side of projection optical system, as well as distortion caused by the projection optical system.
With a recent further progress of fineness tendency of transfer patterns, demands for decreasing the image stress are also becoming greater.
In order to decrease effects of the wafer bowing on the image stress, the conventional technology has employed the so-called image-side telecentric optical system that locates the exit pupil position at a farther point on the image side of projection optical system.
On the other hand, the image stress due to the bowing of reticle can also be reduced by employing a so-called object-side telecentric optical system that locates the entrance pupil position of projection optical system at a farther point from the object plane, and there are suggestions to locate the entrance pupil position of projection optical system at a relatively far position from the object plane as described. Examples of those suggestions are described for example in Japanese Laid-open Patent Applications No. 63-118115 and No. 5-173065 and U.S. Pat. No. 5,260,832.
An object of the invention is to provide an exposure apparatus having a high-performance projection optical system which can correct the aberrations, particularly the distortion, very well even in the bitelecentric arrangement while keeping a relatively wide exposure area and a large numerical aperture.
To achieve the above object, the present invention involves an exposure apparatus having a high-performance projection optical system comprising a stage allowing a photosensitive substrate (for example, a semiconductor wafer coated with a photosensitive material such as a photoresist) to be held on a main surface thereof, an illumination optical system having a light source for emitting exposure light of a predetermined wavelength and transferring a predetermined pattern on a mask onto the substrate, and a projecting optical system for projecting an image of the mask, on the substrate surface. The above projecting optical system projects an image of a first object (for example, a mask with a pattern such as an integrated circuit) onto a second object (for example, a photosensitive substrate).
As shown in FIG. 1, the projection optical system has a first lens group G1 with positive refracting power, a second lens group G2 with negative refracting power, a third lens group G3 with positive refracting power, a fourth lens group G4 with negative refracting power, a fifth lens group G5 with positive refracting power, and a sixth lens group G6 with positive refracting power in the named order from the side of the first object R. The and the second lens group G2 further comprises a front lens L2F placed as closest to the first object R and having negative refracting power with a concave surface to the second object W, a rear lens L2R placed as closest to the second object and having negative refracting power with a concave surface to the first object R, and an intermediate lens group G2M placed between the front lens L2F in the second lens group G2 and the rear lens L2R in the second lens group G2. The intermediate lens group G2M has a first lens LM1 with positive refracting power, a second lens LM2 with negative refracting power, a third lens LM3 with negative refracting power, and a fourth lens LM4 with negative refracting power in the named order from the side of the first object R.
First, the first lens group G1 with positive refracting power contributes mainly to a correction of distortion while maintaining telecentricity, and specifically, the first lens group G1 is arranged to generate a positive distortion to correct in a good balance negative distortions caused by the plurality of lens groups located on the second object side after the first lens group G1. The second lens group G2 with negative refracting power and the fourth lens group G4 with negative refracting power contribute mainly to a correction of Petzval sum to make the image plane flat. The two lens groups of the second lens group G2 with negative refracting power and the third lens group G3 with positive refracting power form an inverse telescopic system to contribute to guarantee of back focus (a distance from an optical surface such as a lens surface closest to the second object W in the projection optical system to the second object W) in the projection optical system. The fifth lens group G5 with positive refracting power and the sixth lens group G6 similarly with positive refracting power contribute mainly to suppressing generation of distortion and suppressing generation particularly of spherical aberration as much as possible in order to fully support high NA structure on the second object side.
Based on the above structure, the front lens L2F placed as closest to the first object R in the second lens group G2 and having the negative refracting power with a concave surface to the second object W contributes to corrections of curvature of field and coma, and the rear lens L2R placed as closest to the second object W in the second lens group G2 and having the negative refracting power with a concave surface to the first object R to corrections of curvature of field, coma, and astigmatism. In the intermediate lens group G2M placed between the front lens L2F and the rear lens L2R, the first lens LM1 with positive refracting power contributes to a correction of negative distortions caused by the second to fourth lenses LM2-LM4 with negative refracting power greatly contributing to the correction of curvature of field.
In particular, in the above projecting optical system, the following conditions (1) to (5) are satisfied when a focal length of the first lens group G1 is f1, a focal length of the second lens group G2 is f2,a focal length of the third lens group G3 is f3, a focal length of the fourth lens group G4 is f4, a focal length of the fifth lens group G2 is f5,a focal length of the sixth lens group G6 is f6,an overall focal length of the second to the fourth lenses LM2-LM4 in the intermediate lens group G2M in the second lens group G2 is fn, and a distance from the first object R to the second object W is L:
0.1 less than f1/f3 less than 17xe2x80x83xe2x80x83(1)
0.1 less than f2/f4 less than 14xe2x80x83xe2x80x83(2)
0.1 less than f5/L less than 0.9xe2x80x83xe2x80x83(3)
0.1 less than f6/L less than 1.6xe2x80x83xe2x80x83(4)
0.1 less than fn/f2 less than 2.0xe2x80x83xe2x80x83(5)
The condition (1) defines an optimum ratio between the focal length f1 of the first lens group G1 with positive refracting power and the focal length f3 of the third lens group G3 with positive refracting power, which is an optimum refracting power (power) balance between the first lens group G1 and the third lens group G3. This condition (1) is mainly for correcting the distortion in a good balance. Below the lower limit of this condition (1) a large negative distortion is produced because the refracting power of the third lens group G3 becomes relatively weak to the refracting power of the first lens group G1. Above the upper limit of the condition (1) a large negative distortion is produced because the refracting power of the first lens group G1 becomes relatively weak to the refracting power of the third lens group G3.
The condition (2) defines an optimum ratio between the focal length f2 of the second lens group G2 with negative refracting power and the focal length f3 of the fourth lens group G1 with negative refracting power, which is an optimum refracting power (power) balance between the second lens group G2 and the fourth lens group G4. This condition (2) is mainly for keeping the Petzval sum small so as to correct the curvature of field well while securing a wide exposure field. Below the lower limit of the condition (2), a large positive Petzval sum appears because the refracting power of the fourth lens group G4 becomes relatively weak to the refracting power of the second lens group G4. Above the upper limit of the condition (2) a large positive Petzval sum appears because the refracting power of the second lens group G2 becomes relatively weak to the refracting power of the fourth lens group G4. In order to correct the Petzval sum in a better balance under a wide exposure field by making the refracting power of the fourth lens group G4 strong relative to the refracting power of the second lens group G2the lower limit of the above condition (2) is preferably set to 0.8, i.e., 0.8 less than f2/f4.
The condition (3) defines an optimum ratio between the focal length f5 of the fifth lens group G5 with positive refracting power and the distance (object-image distance) L from the first object R (reticle or the like) and the second object W (wafer or the like). This condition (3) is for correcting the spherical aberration, distortion, and Petzval sum in a good balance while keeping a large numerical aperture. Below the lower limit of this condition (3) the refracting power of the fifth lens group G5 is too strong, so that this fifth lens group G3 generates not only a negative distortion but also a great negative spherical aberration. Above the upper limit of this condition (3) the refracting power of the fifth lens group G5 is too weak, so that the refracting power of the fourth lens group G4 with negative refracting power inevitably also becomes weak therewith, thereby resulting in failing to correct the Petzval sum well. The condition (4) defines an optimum ratio between the focal length f6 of the sixth lens group G6 with positive refracting power and the distance (object-image distance) L from the first object R (reticle etc.) to the second object W (wafer or the like). This condition (4) is for suppressing generation of higher-order spherical aberrations and negative distortion while keeping a large numerical aperture. Below the lower limit of this condition (4) the sixth lens group G6 itself produces a large negative distortion; above the upper limit of this condition (4) higher-order spherical aberrations appear.
The condition (5) defines an optimum ratio between the overall focal length fn of the second lens LM2 with negative refracting power to the fourth lens LM4 with negative refracting power in the intermediate lens group G2M in the second lens group G2 and the focal length f2 of the second lens group G2. It should be noted that the overall focal length fn, stated herein, of the second lens LM2 with negative refracting power to the fourth lens LM4 with negative refracting power in the intermediate lens group G2M in the second lens group G2 means not only an overall focal length of three lenses, i.e., the second lens LM2 to the fourth lens LM4, but also an overall focal length of three or more lenses between the second lens LM2 and the fourth lens LM4 where there are a plurality of lenses between the second lens and the fourth lens.
This condition (5) is for keeping the Petzval sum small while suppressing generation of distortion. Below the lower limit of this condition (5), a great negative distortion appears because the overall refracting power becomes too strong, of the negative sublens group including at least three negative lenses of from the second negative lens LM2 to the fourth negative lens LM4 in the intermediate lens group G2M in the second lens group G2. In order to sufficiently correct the distortion and coma, the lower limit of the above condition (5) is preferably set to 0.1, i.e., 0.1 less than fn/f2.
Above the upper limit of this condition (5) a great positive Petzval sum results because the refracting power of the negative sublens group including at least three negative lenses of from the second negative lens LM2 to the fourth negative lens LM4 in the intermediate lens group G2M in the second lens group G2 becomes too weak. In addition, the refracting power of the third lens group G3 also becomes weak. Thus, it becomes difficult to construct the projection optical system in a compact arrangement. In older to achieve a sufficiently compact design while well correcting the Petzval sum, the upper limit of the above condition (5) is preferably set to 1.3, i.e., fn/f2 less than 1.3.
Further, the following condition (6) is preferably satisfied when the axial distance from the first object R to the first-object-side focal point F of the entire projection optical system is I and the distance from the first object R to the second object W is L.
1.0 less than I/Lxe2x80x83xe2x80x83(6)
The condition (6) defines an optimum ratio between the axial distance I from the first object R to the first-object-side focal point F of the entire projection optical system and the distance (object-image distance) L from the first object R (reticle or the like) to the second object W (wafer or the like). Here, the first-object-side focal point F of the entire projection optical system means an intersecting point of outgoing light from the projection optical system with the optical axis after collimated light beams are let to enter the projection optical system on the second object side in the paraxial region with respect to the optical axis of the projection optical system and when the light beams in the paraxial region are outgoing from the projection optical system.
Below the lower limit of this condition (6) the first-object-side telecentricity of the projection optical system will become considerably destroyed, so that changes of magnification and distortion due to an axial deviation of the first object R will become large. As a result, it becomes difficult to faithfully project an image of the first object R at a desired magnification onto the second object W. In order to fully suppress the changes of magnification and distortion due to the axial deviation of the first object R, the lower limit of the above condition (6) is preferably set to 1.7, i.e., 1.7 less than I/L. Further, in order to correct a spherical aberration and a distortion of the pupil both in a good balance while maintaining the compact design of the projection optical system, the upper limit of the above condition (6) is preferably set to 6.8, i.e., I/L less than 6.8.
Also, it is more preferable that the following condition (7) be satisfied when the focal length of the third lens L., with negative refracting power in the intermediate lens group G2M in the second lens group G2 is f23 and the focal length of the fourth lens LM4 with negative refracting power in the intermediate lens group G2M in the second lens group G2 is f24.
0.07 less than f24f23 less than 7.xe2x80x83xe2x80x83(7)
Below the lower limit of the condition (7) the refracting power of the fourth negative lens LM4 becomes strong relative to the refracting power of the third negative lens LM3 so that the fourth negative lens LM4 generates a large coma and a large negative distortion. In order to correct the coma better while correcting the negative distortion, the lower limit of the above condition (7) is preferably set to 0.14, i.e., 0.14 less than f24f23. Above the upper limit of this condition (7) the refracting power of the third negative lens LM3 becomes relatively strong relative to the refracting power of the fourth negative lens LM4, so that the third negative lens LM3 generates a large coma and a large negative distortion. In order to correct the negative distortion better while correcting the coma, the upper limit of the above condition (7) is preferably set to 3.5, i.e., f24/f23 less than 3.5.
Further, it is more preferable that the following condition (8) be satisfied when the focal length of the second lens LM2 with negative refracting power in the intermediate lens group G2M in the second lens group G2 is f22 and the focal length of the third lens LM3 with negative refracting power in the intermediate lens group G2M in the second lens group G2 is f23.
0.1 less than f22/f23 less than 10xe2x80x83xe2x80x83(8)
Below the lower limit of the condition (8) the refracting power of the second negative lens LM2 becomes strong relative to the refracting power of the third negative lens LM3, so that the second negative lens LM2 generates a large coma and a large negative distortion. In order to correct the negative distortion in a better balance, the lower limit of the above condition (8) is preferably set to 0.2, i.e., 0.24 less than f22/f23. Above the upper limit of this condition (8) the refracting power of the third negative lens LM3 becomes strong relative to the refracting power of the second negative lens LM2, so that the third negative lens LM3 generates a large coma and a large negative distortion. In order to correct the negative distortion in a better balance while well correcting the coma, the upper limit of the above condition (8) is preferably set to 5, i.e., f22/f23 less than 5.
Also, it is more desirable that the following condition (9) be satisfied when the axial distance from the second-object-side lens surface of the fourth lens LM4 with negative refracting power in the intermediate lens group G2M in the second lens group G2 to the first-object-side lens surface of the rear lens L2R in the second lens group G2 is D and the distance from the first object R to the second object W is L:
0.05 less than D/L less than 0.4.xe2x80x83xe2x80x83(9)
Below the lower limit of the condition (9) it becomes difficult not only to secure a sufficient back focus on the second object side but also to correct the Petzval sum well. Above the upper limit of the condition (9) a large coma and a large negative distortion appear. Further, for example, in order to avoid mechanical interference between a reticle stage for holding the reticle as the first object R and the first lens group G1, there are cases that it is preferable to secure a sufficient space between the first object R and the first lens group G1, but there is a problem that to secure the sufficient space will become difficult above the upper limit of the condition (9).
Also, the fourth lens group G4 preferably satisfies the following condition when the focal length of the fourth lens group G4 is f4 and the distance from the first object R to the second object W is L.
xe2x88x920.98 less than f4/L less than xe2x88x920.005xe2x80x83xe2x80x83(10)
Below the lower limit of the condition (10) the correction of spherical aberration becomes difficult, which is not preferable. Also, above the upper limit of the condition (10), the coma appears, which is not preferable. In order to well correct the spherical aberration and Petzval sum, the lower limit of the condition (10) is preferably set to xe2x88x920.078, i.e., xe2x88x920.078 less than f4/L, and further, in order to suppress generation of coma, the upper limit of the condition (10) is preferably set to xe2x88x920.047, i.e., f4/L less than xe2x88x920.047.
Further, the second lens group G2 preferably satisfies the following condition when the focal length of the second lens group G2 is f2 and the distance from the first object R to the second object W is L.
xe2x88x920.8 less than f2/L less than xe2x88x920.005xe2x80x83xe2x80x83(11)
Here; below the lower limit of the condition (11), a positive Petzval sum results, which is not preferable. Also, above the upper limit of the condition (11), a negative distortion appears, which is not preferable. In order to better correct the Petzval sum, the lower limit of the condition (11) is preferably set to xe2x88x920.16, i.e., xe2x88x920.16 less than f2/L, and in order to better correct the negative distortion and coma, the upper limit of the condition (11) is preferably set to xe2x88x920.0710, i.e., f2/L less than xe2x88x920.0710.
In order to well correct mainly the third-order spherical aberration, it is more desirable that the fifth lens group G5 with positive refracting power have the negative meniscus lens L54, and the positive lens L54 placed adjacent to the concave surface of the negative meniscus lens L54 and having a convex surface opposed to the concave surface of the negative meniscus lens L54 and that the following condition (12) be satisfied when the radius of curvature of the concave surface in the negative meniscus lens L54 in the fifth lens group G3 is r5n and the radius of curvature of the convex surface opposed to the concave surface of the negative meniscus lens L54 in the positive lens L53 set adjacent to the concave surface of the negative meniscus lens L54 in the fifth lens group G5 is r5p.
0 less than (r5pxe2x88x92r5n)/(r5p+r5n) less than 1xe2x80x83xe2x80x83(12)
Below the lower limit of the condition (12), correction of the third-order spherical aberration becomes insufficient; conversely, above the upper limit of the condition (12), the correction of the third-order spherical aberration becomes excessive, which is not preferable. Here, in order to correct the third-order spherical aberration better, the lower limit of the condition (12) is more preferably set to 0.01, i.e., 0.01 less than (r5pxe2x88x92r5n)/(r5p+r5n) and the upper limit of the condition (12) is more preferably set to 0.7, i.e., (r5pxe2x88x92r5n)/(r5p+r5n) less than 0.7.
Here, it is preferred that the negative meniscus lens and the positive lens adjacent to the concave surface of the negative meniscus lens be set between the at least one positive lens in the fifth lens group G5 and the at least one positive lens in the fifth lens group G5. For example, a set of the negative meniscus lens L54 and the positive lens L53 is placed between the positive lenses L52 and L55. This arrangement can suppress generation of the higher-order spherical aberrations which tend to appear with an increase in NA.
Also, it is more desirable that the fourth lens group G4 with negative refracting power have the front lens L41 placed as closest to the first object R and having the negative refracting power with a concave surface to the second object W, the rear lens L44 placed as closest to the second object W and having the negative refracting power with a concave surface to the first object R, and at least one negative lens placed between the front lens L41 in the fourth lens group G4 and the rear lens L41 in the fourth lens group G4 and that the following condition (13) be satisfied when a radius of curvature on the first object side in the rear lens L44 placed as closest to the second object W in the fourth lens group G4 is r4F and a radius of curvature on the second object side in the rear lens L44 placed as closest to the second object W in the fourth lens group G4 is r4R.
xe2x88x921.00xe2x89xa6(r4Fxe2x88x92r4R)/(r4F+r4R) less than 0xe2x80x83xe2x80x83(13)
Below the lower limit of the condition (13), the rear negative lens L44 located closest to the second object W in the fourth lens group G4 becomes of a double-concave shape, which generates higher-order spherical aberrations; conversely, above the upper limit of the condition (13), the rear negative lens L44 located closest to the second object W in the fourth lens group G4 will have positive refracting power, which will make the correction of Petzval sum more difficult.
Further, it is desirable that the fifth lens group G5 have the negative lens L58 with a concave surface to the second object W, on the most second object side thereof. This enables the negative lens L58 located closest to the second object W in the fifth lens group G5 to generate a positive distortion and a negative Petzval sum, which can cancel a negative distortion and a positive Petzval sum generated by the positive lenses in the fifth lens group G5.
In this case, in order to suppress the negative distortion without generating the higher-order spherical aberrations in the lens L61 located closest to the first object R in the sixth lens group G6, it is desirable that the lens surface closest to the first object R have a shape with a convex surface to the first object R and that the following condition be satisfied when a radius of curvature on the second object side, of the negative lens L58 placed as closest to the second object W in the fifth lens group G5 is r5R and a radius of curvature on the first object side, of the lens L61 placed as closest to the first object R in the sixth lens group G6 is r6F.
xe2x88x920.90) less than (r5Rxe2x88x92r5F)/(r5R+r5F) less than xe2x88x920.001xe2x80x83xe2x80x83(14)
This condition (14) defines an optimum shape of a gas lens formed between the fifth lens group G5 and the sixth lens group G6 Below the lower limit of this condition (14) a curvature of the second-object-side concave surface of the negative lens L58 located closest to the second object W in the fifth lens group G5 becomes too strong, thereby generating higher-order comas. Above the upper limit of this condition (14) refracting power of the gas lens itself formed between the fifth lens group G5 and the sixth lens group G6 becomes weak, so that a quantity of the positive distortion generated by this gas lens becomes small, which makes it difficult to well correct a negative distortion generated by the positive lenses in the fifth lens group G5. In order to fully suppress the generation of higher-order comas, the lower limit of the above condition (14) is preferably set to xe2x88x920.30, i.e., xe2x88x920.30 less than (r5Rxe2x88x92r6F)/(r5R+r6F).
Also, it is further preferable that the following condition be satisfied when a lens group separation between the fifth lens group G5 and the sixth lens group G6 is d56 and the distance from the first object R to the second object W is L.
d54L less than 0.017xe2x80x83xe2x80x83(15)
Above the upper limit of this condition (15), the lens group separation between the fifth lens group G5 and the sixth lens group G6 becomes too large, so that a quantity of the positive distortion generated becomes small. As a result, it becomes difficult to correct the negative distortion generated by the positive lens in the fifth lens group G5 in a good balance.
Also, it is more preferable that the following condition be satisfied when a radius of curvature of the lens surface closest m the first object R in the sixth lens group G6 is r6F and an axial distance from the lens surface closest to the first object R in the sixth lens group G6 to the second object W is d6.
0.50 less than d,6/r6F less than 1.50xe2x80x83xe2x80x83(16)
Below the lower limit of this condition (16), the positive refracting power of the lens surface closest to the first object R in the sixth lens group G6 becomes too strong, so that a large negative distortion and a large coma are generated. Above the upper limit of this condition (16), the positive refracting power of the lens surface closest to the first object R in the sixth lens group G61 becomes too weak, thus generating a large coma. In order to further suppress the generation of coma, the lower limit of the condition (16) is preferably set to 0.84, i.e., 0.84 less than d6/r6F.
It is desirable that the following condition (17) be satisfied when the radius of curvature on the first object side in the negative lens L58 located closest to the second object W in the fifth lens group G5 is r5F and the radius of curvature on the second object side in the negative lens L58 located closest to the second object W in the fifth lens group G5 is r5R.
0.30 less than (r5Fxe2x88x92r5R)/(r5F+r5R) less than 1.28xe2x80x83xe2x80x83(17)
Below the lower limit of this condition (17), it becomes difficult to correct both the Petzval sum and the coma; above the upper limit of this condition (17), large higher-order comas appear, which is not preferable. In order to further prevent the generation of higher-order comas, the upper limit of the condition (17) is preferably set to 0.93, i.e., (r5Fxe2x88x92r5R)/(r5F+r5R) less than 0.93.
Further, it is desirable that the second-object-side lens surface of the first lens LM1 with positive refracting power in the intermediate lens group G2M in the second lens group G2 be of a lens shape with a convex surface to the second object W, and in this case, it is more preferable that the following condition (18) be satisfied when the refracting power on the second-object-side lens surface of the first positive lens LM1 in the intermediate lens group G2m in the second tens group G2 is "PHgr"21 and the distance from the first object R to the second object W is L.
0.54 less than 1/("PHgr"21xc2x7L) less than 10xe2x80x83xe2x80x83(18)
The refracting power of the second-object-side lens surface, stated herein, of the first lens LM1 with positive refracting power in the intermediate lens group G2M is given by the following formula when a refractive index of a medium for the first lens LM1 is n1, a refracting index of a medium in contact with the second-object-side lens surface of the first lens LM1 is n2, and a radius of curvature of the second-object-side lens surface of the first lens is r21.
"PHgr"21=(n2xe2x88x92n1)/r21
Below the lower limit of the condition (18), higher-order distortions appear; conversely, above the upper limit of the condition (18), it becomes necessary to correct the distortion more excessively by the first lens group G1, which generates the spherical aberration of the pupil, thus being not preferable.
Further, it is more preferable that the following condition (19) be satisfied when the focal length of the first lens LM4 with positive refracting power in the intermediate lens group G2M in the second lens group G2 is f21 and the distance from the first object R to the second object W is L.
0.230 less than f less than /L less than 0.40xe2x80x83xe2x80x83(19)
Below the lower limit of the condition (19), a positive distortion appears; above the upper limit of the condition (19), a negative distortion appears, thus not preferable.
Also, the front lens L2F and rear lens L2R in the second lens group G2 preferably satisfy the following condition when the focal length of the front lens L2F placed as closest to the first object R in the second lens group G2 and having the negative refracting power with a concave surface to the second object W is f2F and the focal length of the rear lens L2R placed as closest to the second object W in the second lens group G2 and having the negative refracting power with a concave surface to the first object R is f2R.
0f2F/f2R less than 18xe2x80x83xe2x80x83(20)
The condition (20) defines an optimum ratio between the focal length f2R of the rear lens L2R in the second lens group G2 and the focal length r2F of the front lens L2F in the second lens group G2. Below the lower limit and above the upper limit of this condition (20), a balance is destroyed for refracting power of the first lens group G1 or the third lens group G3, which makes it difficult to correct the distortion well or to correct the Petzval sum and the astigmatism simultaneously well.
The following specific arrangements are desirable to provide the above respective lens groups with sufficient aberration control functions.
First, in order to provide the first lens group G1 with a function to suppress generation of higher-order distortions and spherical aberration of the pupil, the first lens group G1 preferably has at least two positive lenses; in order to provide the third lens group G3 with a function to suppress degradation of the spherical aberration and the Petzval sum, the third lens group G3 preferably has at least three positive lenses; further, in order to provide the fourth lens group G4 with a function to suppress the generation of coma while correcting the Petzval sum, the fourth lens group G4 preferably has at least three negative lenses. Further, in order to provide the fifth lens group G5 with a function to suppress generation of the negative distortion and the spherical aberration, the fifth lens group G5 preferably has at least five positive lenses; further, in order to provide the fifth lens group G5 with a function to correct the negative distortion and the Petzval sum, the fifth lens group G5 preferably has at least one negative lens. Also, in order to provide the sixth lens group G6 with a function to converge light on the second object W without generating a large spherical aberration, the sixth lens group G6 preferably has at least one positive lens.
In addition, in order to correct the Petzval sum better, the intermediate lens group G2 in the second lens group G2 preferably has negative refracting power.
In order to provide the sixth lens group G6 with a function to further suppress the generation of the negative distortion, the sixth lens group G6 is preferably constructed of three or less lenses having at least one surface satisfying the following condition (21).
1/|"PHgr"L| less than 20xe2x80x83xe2x80x83(21)
where "PHgr": refracting power of the lens surface;
L: object-image distance from the first object R to the second object W.
The refracting power of the lens surface stated herein is given by the following formula when the radius of curvature of the lens surface is r, a refracting index of a medium on the first object side, of the lens surface is n1, and a medium on the second object side, of the lens surface is n2.
"PHgr"=(n2xe2x88x92n1)/r
Here, if there are four or more lenses having the lens surface satisfying this condition (21), the number of lens surfaces with some curvature, located near the second object W, becomes increased, which generates the distortion, thus not preferable.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art form this detailed description.